1. Technical Field
The present invention relates to a novel polishing slurry that can be favorably used for polishing semiconductor wafers as represented by silicon and for polishing an inter-layer dielectric in the IC fabrication process (hereinafter often referred to as CMP). More specifically, the invention relates to a polishing slurry comprising a silica-dispersed solution obtained by dispersing fumed silica particles in an aqueous solvent, featuring a high purity, excellent polishing performance and excellent preservation stability.
2. Background Art
A conventional polishing slurry comprising a silica-dispersed solution can be represented by the one which comprises colloidal silica obtained by using sodium silicate as a starting material. For example, New Ceramics No. 2, pp. 45-46, 1995 discloses a polishing slurry for silicon wafers comprising colloidal silica obtained from sodium silicate. The colloidal silica is synthesized in a liquid phase and is used in the form of a dispersion without being dried, exhibiting small average particle diameter, good dispersion and very good stability in a solution.
Generally, however, the colloidal silica has a low purity and could not be used as a polishing slurry for semiconductor wafers that require high purity or as a polishing slurry for the inter-layer dielectric in the IC process. In particular, the polishing slurry for the inter-layer dielectric causes a serious problem of contamination due to cations such as of heavy metals and, hence, must have a high purity.
To meet these requirements, the above literature proposes a method of obtaining colloidal silica by using an organosilicon compound such as alkoxide as a starting material of colloidal silica.
However, the colloidal silica is produced on an industrial scale maintaining a poor yield. Besides, the method which uses the organosilicon compound involves a problem of an increase in the cost of production due to the starting material.
Moreover, the particles of the colloidal silica are soft and easily adhere strongly on the polished surfaces and cannot be removed to a sufficient degree even by the washing. This means that the polished surfaces are further contaminated when use is made of the colloidal silica containing much impurities, that is obtained by using the sodium silicate as a starting material.
In addition to the colloidal silica, there has been known a fumed silica which is a fine silica and is obtained on an industrial scale maintaining a good yield by burning a silicon tetrachloride in the hydrogen-oxygen combustion flame. The fumed silica has a primary particle size which is as small as that of the colloidal silica and has a high purity. Therefore, attempts have been made to use the fumed silica as a polishing slurry to substitute for the polishing slurry which uses the colloidal silica.
For example, Japanese Patent Publication No. 20350/1982 and Japanese Laid-Open Patent Publication No. 60420/1991 disclose a silicon-dispersed solution obtained by dispersing a fumed silica in an aqueous solvent and a polishing slurry for silicon wafers comprising the above silica-dispersed solution.
However, the fumed silica is provided in the form of a dry aggregated powder which, when it has a large specific surface area (small primary particle size), can be stably dispersed with very difficulty in an aqueous solution as fine particles.
The above-mentioned literature therefore teaches that it is desirable to use fumed silica having a specific surface area of not larger than 75 m.sup.2 /g to produce a silica-dispersed solution in which silica particles are favorably dispersed.
The fumed silica having a specific surface area of not larger than 75 m.sup.2 /g has an average primary particle size of as relatively large as not smaller than 36 nm as reckoned from the specific surface area that will be described later, and makes it relatively easy to obtain a silica-dispersed solution having good dispersion property. However, the fumed silica that is formed in the flame has hard particles. When the silica-dispersed solution of the fumed silica is used as a polishing slurry, therefore, the polished surfaces tend to be scarred and lose planarity, arousing a problem from the standpoint of using it as a polishing slurry for the semiconductor wafers or for the inter-layer dielectric during the IC process. In the CMP for polishing the inter-layer dielectric (often called wiring layer) as represented by a glass film in the IC process that is drawing attention in recent years, in particular, scars formed on the wiring layer are causing a breakage of the wiring formed thereon. According to the present inventors, furthermore, it has been confirmed that the supernatant liquid forms after the silica-dispersed solution is left to stand for several months; i.e., the silica-dispersed solution has poor preservation stability.
When it is attempted to prepare a silica-dispersed solution using a fumed silica having a small primary particle size and a specific surface area of not smaller than 75 m.sup.2 /g by using a high-shearing mixer that has heretofore been used for the dispersion, poor dispersion stability is obtained since silica has poor dispersion property. Besides, the silica-dispersed solution becomes no longer utilizable as its viscosity increases. Even if the silica-dispersed solution having a suitable viscosity is obtained, the polishing slurry comprising such a silica-dispersed solution contains particles of various sizes made up of primary particles that are aggregated. Therefore, the polishing performance is not favorably reproduced and, besides, the polishing rate is low.
In order to improve stability of the silica-dispersed solution using the fumed silica, furthermore, Japanese Patent Publication No. 338/1993 discloses a water dispersion composition (silica-dispersed solution) of silicic anhydride obtained by dispersing, in an aqueous solvent, a silicic anhydride having a surface silanol group density of not smaller than 0.3 but not larger than 3 per 1 nm.sup.2.
Though there has been described that the silica-dispersed solution has good stability, the silica-dispersed solution is obtained by drying the silica before being dispersed and treating the surfaces of the silica with an alkoxysilane coupling agent to satisfy the above-mentioned conditions, requiring cumbersome operation. Besides, the dispersion machine used for the method of producing the dispersion solution of the surface-treated silica is of the high-shearing type like the one used in the above-mentioned prior art. The silica-dispersed solution obtained by using the above dispersion machine contains aggregated particles of various sizes as described above, and still leaves much room for improvement concerning its polishing performance so that it can be used as a polishing slurry.